Reflectance ratio is the ratio of the intensity of light reflected from a sample to the intensity of light incident on the sample. It is relatively easy to measure the intensity of light reflected from a sample, however, measuring the light incident on the sample is more difficult. Incident light has been measured by placing a primary reflectance standard in a readhead of a spectrophotometer and, by knowing the true reflectance at the operational wavelength of the device, the incident light can be back-calculated from the detected deflectance. Reflectance of the test sample then is determined from the intensity of light reflected from the test sample divided by the intensity of light reflected from the primary reflectance standard multiplied by the known reflectance of the reflectance standard. This technique assumes that there is no stray light within the reflectance measuring device, but this assumption is wrong and leads to significant measurement errors.
To calibrate reflectance measuring devices, such as spectrophotometers, it is customary to establish 100% and 0% reflectance values. To establish these reflectance values, a measurement relative to some standard must be made. The 0% reflectance value has been difficult to obtain, and many procedures and devices operate on the assumption that 0% reflectance is the value measured with the device turned off. The resulting 0% reflectance value is not correct since it is very dfficult to impossible to eliminate stray light.
In the prior art, recalibration using a primary reflectance standard must be performed before each measurement and the instrument sensitivity or gain must be reset. Under these conditions, each measurement and reading is dependent on the sensitivity or gain set of the device. The necessity for the user to place a primary reflectance standard into the device prior to each measurement to reset the instrument sensitivity decreases the utility of the device and increases the cost of use. Further, each reflectance measuring device must be made to close tolerances to insure that the stray light for each device is the same. This requires the maintenance of costly and difficult manufacturing tolerances and strict quality control standards. Another disadvantage with these devices is a need for frequent device recalibration due to constant sensitivity changes.
One approach to calibrating spectrophotometers is disclosed in U.S. Pat. No. 4,029,419. In this system, the spectrophotometer is calibrated by using a secondary white standard calibrated against a primary white standard, a black standard to compersate for dark current and internal reflectance from a pressure plate, and a mirror to obtain internal wall reflectance used in a correction factor. Although this approach does not assume that the stray light is zero, for each use of the spectrophotometer two samples must be read and the instrument recalibrated.
A process of compensating for radiation variances in a spectrophotometer light source is disclosed in U.S. Pat. No. 3,245,305. The spectrophotometer described in this patent employs two light sources and means for sensing the relative intensities of the radiation from the two sources. The device of U.S. Pat. No. 3,245,305 also includes means for altering the intensity of the radiation from one light source. Additional apparatus is provided to be responsive to the sensing apparatus for automatically changing the intensity of the radiation from one light source to keep the ratio of the intensity of the two sources constant. This device does not measure the incident light directly. Further, the requirement of a second light source and the attendant electronics increases the complexity and cost of the device and its use.
U.S. Pat. No. 3,646,331 discloses a method and apparatus for correcting radiation measurment errors in a spectrophotometer by digitizing the output of a 100% line input at selected discrete wavelengths. A factor, called a M factor, is computed from the output at each discrete wavelength such that the digitized output multiplied by the M factor will give a corrected 100% output at each wavelength. These M factors are each stored, and during a sample measurement by the spectrophctometer, each of the stored M factors is synchronously applied to multiply the input signal derived from the signal, thereby generating a corrected output. This apparatus and method completely neglects the 0% reflectance value, whereby a significant, inherent error is introduced into the apparatus and method.
U.S. Pat. No. 4,310,243 discloses a spectrophotometer and a method of simultaneously compensating for the dark current of a photomultiplier tube and the offset of an operational amplifier in the spectrophotometer, so that the output voltage of the operational amplifier is zero volts under dark conditions. Although this patent discloses a technique for compensating for the dark signal of a detector, it does not account for stray light.